1. Field of the Invention
The present invention relates to a gas sensor for measuring gas components such as NO, NO.sub.2, SO.sub.2, CO.sub.2, and H.sub.2 O contained in, for example, atmospheric air and exhaust gas discharged from vehicles or automobiles. In particular, the present invention relates to a structure of a protective cover which is arranged to surround a sensor element.
2. Description of the Related Art
A variety of gas sensors have been suggested and practically used until the present, including, for example, oxygen sensors, NOx sensors (see Japanese Laid-Open Patent Publication No. 8-271476), and HC sensors (see Japanese Laid-Open Patent Publication No. 8-247995) based on the use of oxygen ion conductors; hydrogen sensors and H.sub.2 O sensors based on the use of proton ion conductors; and oxygen sensors and various gas sensors based on the use of oxide semiconductors such as SnO.sub.2 and TiO.sub.2.
Among the gas sensors described above, the oxygen sensor based on the use of ZrO.sub.2 and the oxygen sensor based on the use of TiO.sub.2 are widely used for A/F control or for controlling the oxygen concentration in exhaust gas discharged from vehicles or automobiles, because they still maintain stable performance even in an environment of automobile exhaust gas.
The NOx sensor based on the use of ZrO.sub.2 has also arrived at the stage of practical use for controlling NOx for automobiles.
In general, the gas sensor based on the use of ZrO.sub.2 contains a heater. An electric power is applied to a heater simultaneously with the start of the engine. The temperature of the sensor element is raised earlier than the temperature of exhaust gas to be raised, and it arrives at an operation temperature.
On the other hand, condensed water is produced during the starting operation of the engine. The amount of production of the condensed water decreases as the temperature of exhaust gas is raised, and the condensed water gradually disappears. Therefore, when the sensor element is sufficiently heated during the time zone of condensed water production, a thermal shock may be exerted on the sensor element due to adhesion of the condensed water, resulting in occurrence of cracks.
Especially, the gas sensor, in which the heater and the sensor element are integrated into one unit, has suffered from a drawback that the crack highly probably occurs, because the temperature of the sensor is quickly raised, and the sensor is sufficiently heated to a high temperature during the time zone of condensed water production.
In order to dissolve the drawback described above, a protective cover has been suggested as a countermeasure against condensed water for the oxygen sensor of the type containing a heater therein, and it is practically used at present (for example, see Japanese Laid-Open Patent Publication No. 5-26842).
However, in recent years, since these gas sensors come to be attached downstream of the three way catalyst, involves a problem that the time zone of condensed water production is markedly prolonged, and the probability of crack formation in the sensor element is increased even when the foregoing protective cover is used, as compared with the conventional gas sensor which is attached upstream of the three way catalyst.
Further, those based on the use of the oxygen pump function, such as the NOx sensor, are heated to a temperature not lower than 700.degree. C. in order to effectively operate the oxygen pump. Therefore, they involve a problem that the thermal shock exerted thereon upon contact with condensed water is by far greater than that exerted on the oxygen sensor of the heating type, and the probability of crack formation is high.
Furthermore, in the case of the gas sensor based on the use of the sensor element which should be heated to a high temperature as described above, the low temperature environment is dominant as the sensor is attached to a more downstream position in an exhaust pipe. As a result, an inconvenience arises in that the sensor element tends to be cooled, and the oxygen pump is not operated effectively. Especially, this problem is serious for the diesel engine and the lean burn engine, because the exhaust gas temperature is low.